The present invention relates to the use of a vegetable filler of low density and controlled particle size. It relates more particularly to the preparation of a composite sheet material, containing fibers, which is more porous and has a reduced density with an improved bulk by virtue of such a vegetable filler. It is of particular value in the field of the manufacture of fibrous sheets such as paper, board and other nonwovens.
It is conventional in papermaking to use mineral, organic or synthetic fillers or pigments. The most commonly used mineral fillers and pigments are talc, kaolin, natural and precipitated calcium carbonate or calcium carbonate originating from the operations for regenerating the black liquor extracted from the digestion of kraft pulps, magnesium carbonate, alumina hydrates, calcium sulfate, colloidal silica, barium sulfate, titanium dioxide, magnesium hydroxide and mixtures thereof.
The conventional fillers used in papermaking have variable equivalent spherical diameters, measured by means of the SEDIGRAPH device, but as a general rule all these fillers have particles with dimensions smaller than 100 micrometers, in particular smaller than 50 micrometers and especially smaller than 30 micrometers.
Thus 50% of the particles of the kaolin to be incorporated into the mass have a size of between 6 and 8 micrometers (d.sub.50 =6-8 micrometers) and 95% of the particles have a size of less than 50 micrometers (d.sub.95 &lt;50 micrometers). The talc to be incorporated into the mass has a d.sub.50 of between 8 and 10 micrometers and a d.sub.95 of less than 50 micrometers and the calcium carbonate has a d.sub.50 of between 7 and 9 micrometers and a d.sub.95 of less than 25 micrometers.
The surface-coating pigments have a d.sub.50 of between 1 and 3 micrometers and a d.sub.95 of less than 10 micrometers. The organic pigments are much finer and have a d.sub.50 of between 0.1 and 1 micrometer.
It is known, especially from EP-B-0 006 390, EP-B-0 050 316 and FR-A-2 578 870, that mineral fillers favor a lowering of the material cost and, depending on their nature, favor the opacity, in particular in the sector of printing-writing substrates and certain special types of paper. If the proportion of mineral fillers in the material is high compared with that of the cellulosic fibers, they can favor the dimensional stability, which is particularly desirable in bases for floor or wall covering; however, on the one hand the use of these mineral fillers has an adverse effect on the thickness of the material, and on the other hand the increase in the residual ash content also results in a substantial increase in the density of the material, which does not favor the cost per m.sup.2.
The paper industry uses certain special fillers or pigments to increase the thickness of the sheet, for example diatomite or natural, calcined or activated diatomaceous earths, carbon or glass microspheres, calcium silicates and colloidal silicas, but these fillers or pigments are generally very expensive.
The incorporation of so-called mechanical cellulosic pulps, such as thermomechanical or chemicomechanical pulps, and, to a lesser extent, mercerized pulps and synthetic fibers, especially polyester, polyethylene and polypropylene fibers, also plays a part in improving the bulking (or body of paper or puffing out), but the use of these fibers demands substantial industrial means and only concerns very specific applications.
It is known that the possibility of using wood powder or flour as a filler, introduced into the mass or deposited on the surface by coating, in the manufacture of paper and board has already been indicated in the past.
It is known in particular that abstract no. 8739 of the journal ABSTRACT BULLETIN OF THE INSTITUTE OF PAPER CHEMISTRY, 48, (no. 8), page 938, (February 1978), U.S. Pat. No. 3,184,373 and German patent DE-C-415 675 provide for the incorporation of wood powder into the fibrous mass.
Abstract no. 8739 mentioned above recommends the use of a mixture of 70-95% by weight of cellulosic pulp (kraft fibers) and 30-5% by weight of wood flour for the manufacture of electrically insulating panels, these panels being said to be more absorbent towards oils and more resistant to surface discharges. The said document neither describes nor suggests the use of a vegetable filler having the particle size and density specific to the present invention.
U.S. Pat. No. 3,184,373 relates to the improvement in the retention of fillers in paper and board by means of a retention agent such as polyethyleneimine, melamine-formaldehyde resins and urea-formaldehyde resins, the said "fillers" being defined (see column 2, lines 3-34) as being solid or liquid substances and comprising, in particular, mineral paper fillers themselves, metal powders, thermosetting resin powders thermoplastic resins, binders, flocculants and wood powder (see column 2, line 27). The particle size of the said "fillers" is said to be between 60 mesh and 2000 mesh (see column 1, lines 70-71). However, U.S. Pat. No. 3,184,373 does not give any Examples illustrating the use of the said wood powder introduced into the mass; furthermore, it neither describes nor suggests the particle size (particle dimensions of between 10 and 150 micrometers) and the density (less than 500 kg/m.sup.3) specific to the vegetable filler according to the invention.
German patent DE-C-415 675 proposes a sizing process in which (i) a colloidal dispersion of a submicron substance (i.e. a substance with a particle size of less than 1 micrometer) containing cellulose and originating from the grinding of wood or straw, and then (ii) a flocculant, are incorporated into an aqueous suspension of fibers. The said submicron substance, which therefore has a particle size well below that of the vegetable filler according to the invention, fulfils a totally different role from that of the said vegetable filler; in fact, the said submicron substance is presented in German patent document C-415 675 as reducing the porosity of the resulting paper by blocking and/or filling the pores in the fibrous web, whereas an increase in the porosity is sought according to the invention.
Techniques of covering (on a non-fibrous substrate) or surface sizing (on a fibrous sheet substrate such as paper and board) are known from other sources, especially from Belgian patent BE-A-425 432, published PCT International application WO 86/05195 and British patent GB-A-1 464 381, wherein a substrate is coated with a composition containing wood powder. It so happens that these techniques neither describe nor suggest the incorporation, into the fibrous mass, of the vegetable filler of specific particle size and density according to the invention.
Products which are capable of being incorporated into paper and board, contain cellulose and are obtained by the physicochemical treatment of wood chips or cellulosic fibers are also known, especially from abstract no. 1523 of the journal ABSTRACT BULLETIN OF THE INSTITUTE OF PAPER CHEMISTRY, 58 (no. 2), page 184 (August 1987), abstract no. 7191, ibidem 55 (no. 6), page 754, (December 1984), and published French patent application FR-A-2 612 828 (which was made public on Sep. 30, 1988, after the priority date of the present invention). The composition of the said products containing cellulose is different from that of the vegetable or fibrous source from which they are derived. In fact, the physical and chemical treatments to which the said source has been subjected do not preserve the integrity of the components of the said source.
More precisely, abstract no. 1523 mentioned above describes the preparation of cellulose in the form of micronized particles (having a size of between 5 and 75 micrometers and a degree of crystallinity greater than 65%) by the hydrolytic treatment of cellulosic pulps. The cellulose obtained in this way differs from the composition of the vegetable filler according to the invention in respect of the nature of the components of the latter.
Abstract no. 7191 mentioned above describes the use of microfibrillated cellulose for the production of coatings for covering. Here again the microfibrils of cellulose differ in their structure and composition from the vegetable filler according to the invention.
French patent document FR-A-2 612 828 is deceptive in the sense that its claim 1, as published, refers to the use of wood particles in the preparation of fibrous sheets, whereas what is actually involved is the use of an extract, in pulverulent form, obtained by treating wood powder, the said treatment comprising the following steps in particular (see descriptive part of the said document from page 1, line 28, to page 2, line 12): (i) impregnation of the wood powder with an appropriate liquid chemical agent (sic), (ii) flash autolysis (or flash autohydrolysis) of the impregnated wood powder under a pressure greater than or equal to 30 bar, at a temperature greater than or equal to 230.degree. C., for at least 90 seconds, followed by a rapid (sudden) release of pressure, (iii) washing of the resulting product with water or a water/dioxane mixture in order to remove the hemicelluloses and most of the lignin, fatty acids and resin acids, and then (iv) drying of the extracted pulverulent product containing water-insoluble materials and devoid of water-soluble materials.
The present invention differs from the teaching of the said abstracts no. 1523 and no. 7191 and the said French patent document FR-A-2 612 828 by the fact that the vegetable filler, which it is recommended to use in the mass, has preserved substantially all the components of the vegetable source; in the vegetable filler according to the invention, only the water content and the content of volatile substances (such as the low-boiling essential oils) have been affected by comparison with the starting vegetable source. Thus, if the vegetable source is wood, practically all the components of the wood, as described in the work by FENGEL et al., WOOD CHEMISTRY ULTRASTRUCTURE REACTIONS, pages 26-33, published by D. GRUYTER (1984), which is incorporated here by way of reference, will be found in the vegetable filler.
Referring to the teaching of the nearest prior art, which consists of abstract no. 8739 of the journal ABSTRACT BULLETIN OF THE INSTITUTE OF PAPER CHEMISTRY, 48, (no. 8), page 938, (February 1978), U.S. Pat. No. 3,184,373 and German patent DE- C-415 675 mentioned above, it should be pointed out that the use of wood flour or powder has never worked on the industrial scale according to precise specifications (i) for the density and particle size of the vegetable filler, and (ii) for the purpose of obtaining, by a papermaking technique, sheets of fibrous materials which are more economical and have a lower density, with a high porosity and a high bulk.